Various thermoplastic elastomeric compositions are well-known and include thermoplastic urethanes, thermoplastic polyesters, amorphous polypropylenes, chlorinated polyethylenes, ethylene/propylene rubbers, crosslinked and uncrosslinked EPDM (ethylene-propylene-nonconjugated diene monomer) systems, and styrene block copolymers. Styrene block copolymers, which are sold under the brand names of Vector, Kraton and Solprene as supplied respectively by Dexco Polymers, Shell Oil Company and Phillips Petroleum, are considered to be very versatile thermoplastic elastomers.
Styrene block copolymers are also recognized as strong, flexible high performance elastomers that do not require vulcanization and yet exhibit excellent elasticity over a wide range of service temperatures. Due to their unique molecular structure and versatility, styrene block copolymers are used in a wide spectrum of enduses such as moldable goods, automotive interior and exterior parts, medical devices, and the like.
Styrene block copolymers are available with linear, diblock, triblock and radial molecular structures. Each polymer molecule consists of a styrenic block segment and a rubber monomer block segment. The rubber segment may consist of saturated or unsaturated monomer units such as ethylene/butene and ethylene/propylene, or butadiene and isoprene, respectively. Styrene block copolymers having saturated rubber monomer segments typically exhibit improved thermal, oxidative and processing stability, better weather resistance, and higher temperature serviceability when compared to copolymers consisting of unsaturated rubber monomer segments.
Although styrene block copolymers are very useful, they have a number of deficiencies. These materials are relatively expensive and due to their fairly difficult manufacturing requirements are often in short supply. Moreover, due to the inability of their particles or pellets to expediently melt and fuse together with the application of heat, styrene block copolymers with saturated rubber monomer segments are fairly difficult to formulate and process. Processing in conventional equipment such as, for example, a Banbury mixer, is typically characterized by an initial induction or delay period which adds to processing costs. As another disadvantage, this delay, which in effect constitutes an additional heat history, can contribute to the overall thermal and processing instability of the copolymer.
Because of high material costs, shortages and processing difficulties, it is desirable to provide blend components that can extend available quantities of styrene block copolymers without substantially altering the key elastic properties of the latter. It is also desirable to provide component materials that function as processing aids or fusion promoters whereby the delay times associated with thermally processing styrene block copolymers having saturated rubber monomer segments can be substantially reduced. Further, it is desirable to improve the properties of block copolymers having unsaturated rubber monomer block segments without the higher manufacturing costs typically associated with block copolymers having saturated rubber monomer segments.
There has been a long felt need to mitigate the above difficulties associated with conventional thermoplastic elastomeric materials but, unfortunately, prior art efforts to do so have not been entirely successful. Prior art proposals to facilitate the use of ordinary thermoelastic materials tend to involve curing steps or multicomponent compositions with extensive formulating requirements. For example, Shell Oil Company in its Kraton brochure indicates the Kraton materials are highly extendable presumably by specific combinations of involving fillers, resins and oils. On page 3 of the brochure, Kraton D compounds and Kraton G compounds are said to contain other suitable ingredients. Further, where other ingredients are used, including fillers and oils in combination or alone, it is expected that special handling and equipment would be required for uniform admixing with solid block copolymer resins.
Where ordinary thermoelastic materials, such as, for example, ethylene/vinyl acetate (EVA) copolymers, are used as single-component extenders for block copolymers, the elastic, rheological, stability or hardness properties of the final composition tend to vary substantially relative to neat compositions of the respective block copolymer. Still other prior art disclosures involving ethylene/.alpha.-olefin interpolymers in combination with block copolymers such as, for example, U.S. Pat. No. 5,272,236, which discloses blends of substantially linear ethylene interpolymers and styrene butadiene copolymers, and Plastics Technology, August 1994, page 54, which mentions similar blends useful for molded goods, do not teach or render obvious the specific requirements that enable the use of such materials as substantially inert extenders, nor disclose the surprising benefits that can be realized by doing so.